The receiving server refused your message on content or reputation grounds.
READ THE FULL BOUNCE
It usually names the reason, and frequently includes a reference or a link to the blocking organisation's page.
That is the fastest route to the actual cause.
THE COMMON REASONS
SPF, DKIM or DMARC failing Your domain or sending address on a blacklist The From address not on your own domain
Content resembling marketing: many links, image-heavy, aggressive subject
Sending through PHP mail rather than authenticated SMTP
THE FIRST FIX
cPanel > Email Deliverability. Check SPF and DKIM for the domain, and repair where indicated.
Authentication problems account for most of these, and nothing else helps while they are wrong.
IF IT AFFECTS ONE RECIPIENT ONLY
Their filter or their server policy. Ask them to whitelist you.
IF IT AFFECTS EVERYONE
A reputation or authentication problem at your end.
CHECK BLACKLISTS
Look up your domain and sending address. If listed, fix the cause before requesting delisting.
